The present invention relates to self-powered neutron detectors. A self-powered neutron detector basically consists of a neutron responsive emitter core, an insulator layer which retains high electrical resistivity even when continuously exposed to intense radiation fields, and a conductive collector layer which produces few electrons or gamma rays in a neutron flux as compared with the emitter core material. The detector is termed self-powered because there is no need to impose an operating voltage across the emitter and conductor electrodes. Neutrons are absorbed by the emitter and give rise to an electron current between the emitter and the collector which is externally measured as the detector signal current.
Nuclear reactor in-core safety systems require prompt response miniature neutron detectors for measurements of variations in local power densities. Self-powered detectors utilizing cobalt as the emitter material for a prompt response characteristic have been used for in-core safety applications. The cobalt emitter core of such self-powered detectors utilizes the captured gamma rays which result from the absorption of an incident neutron by a cobalt nucleus in the emitter core. The resulting outward flow of these gamma rays produces, by means of interactions with the detector material, a net outward flow of high average energy prompt electrons. This displacement of charge results in a current flow between the emitter and collector which is externally measured in a high sensitivity ammeter. The current produced is proportional to the instantaneous neutron flux. A sensitivity problem exists for such detectors where the emitter material has activation products which decay with time, such as a cobalt emitter detector which exhibits a build up of cobalt activation products with time of exposure. These cobalt activation products emit low average energy beta electrons and also gamma rays which cause a delayed current background signal that increases with detector irradiation.
An improved self-powered prompt response detector which minimizes the effect of such delayed currents produced from activation products is described in U.S. Pat. No. 3,872,311, owned by the assignee of the present invention. The teaching of the aforementioned patent is to provide a thin conductive layer of low neutron cross section, high density material about the emitter core material. This high density layer absorbs beta radiation emitted by the emitter core activation products, but is substantially transmissive to the high average energy prompt electrons emitted by the emitter core material. The materials which have been suggested for use as the conductive, low average energy beta absorptive layers are platinum, bismuth and lead. These materials even in a thin layer have the effect of increasing the sensitivity of the device to external gamma rays, and thereby decrease the neutron to gamma signal ratio for the detector. It is desirable to compensate for the increased gamma sensitivity of the conductive low average energy beta absorptive layer so that the detector is primarily neutron responsive.